8 hour sleeping time

My son ho, really only can stand 8 hours sleeping time only. No matter what time he start to sleep, then i just need to add another 8 hours - that's the time he will wake up and ask for nen nen. So he will: "mama, nen nen. Mama, nen nen." I still didn't turn up in front of him, he will start cry la. After nen nen is his playing time again lo. So every morning, sometime will be quite rush for me. I have to make sure i wake up at 630 am then only i can be in time. But my Jing pulak, she won't wake up until she is "fully" enough sleep. Even when she is hungry, she will ask for nen nen but will go back to sleep after nen nen. Both my children have totally different character.